This was happening to me after a re-install of Visual Studio Community 2022 and just could not get tests to run.

The problem was that Microsoft had omitted from the build all of the unsupported frameworks, including .NET 6.0.

These are the steps for the fix:

1. Open the Visual Studio Installer and choose 'Modify'
2. On Components page, click the tab (at the top) 'Individual Components'
3. Choose from the list the framework(s) you want included, for me it was .NET 6.0 Runtime.
4. Click Install while downloading. 5 Once installed, Start VS and rebuild your solution.
5. All tests appear and run!!